Before mounting a pair of ophthalmic lenses for correcting eyeglasses in a frame, an optician generally seeks to verify that the main optical characteristics of the lenses, and in particular their refractive characteristics, at one or more points do indeed correspond to the prescription for which they were ordered. At present, this can consist, for example, in verifying the spherical, cylindrical, and prismatic powers and the cylindrical and prismatic orientations at one or more prescribed points. When the prescribed lens is a single-vision lens, the prescribed point where measurement is to be performed is the optical center of the lens. When the lens is bifocal, the prescribed points are the reference points for near vision and for far vision, in positions that are defined by the manufacturer in the frame of reference marked on the lens. More generally, it can be desired to verify the refractive characteristics of the lens at one or more points of interest at any positions that are determined in the frame of reference of the lens.
The refractive characteristics of a lens are indeed written by the manufacturer on the envelope in which each lens is delivered. However numerous opticians prefer to verify compliance of the actual main refractive characteristics of each lens immediately before mounting in order to eliminate any risk of error. It can also happen that the envelope bearing the prescription of a lens is missing, for example when it is desired to discover or verify the characteristics of a lens that has already been mounted or prepared for mounting.
In addition, prior to the optician mounting a lens on a frame, a lens manufacturer can also desire to verify the optical characteristics of manufactured lenses, or indeed to measure the optical characteristics of lenses from competitors.
In order to undertake such measurements and/or verifications, opticians and manufacturers thus require apparatus capable of measuring the main refractive characteristics of a lens at one or more defined points thereof.
Several categories of apparatus are known that satisfy this need more or less well. In the context of the present invention, existing apparatuses can be classified in two main categories. Thus, firstly there are narrow-field measurement apparatuses having a single measurement axis that is stationary and that are designed to perform targeted measurement on a measurement zone of small size around the measurement point, and secondly there are wide-field mapping apparatuses designed to perform measurement that is complete, or at least that extends over a large area of the lens, at a large number of points simultaneously.
Amongst narrow-field measurement apparatuses, there can be included apparatuses that have been in widespread use for many years and that are known as frontofocometers. One of the first apparatus of that kind is described in U.S. Pat. No. 1,383,678. The principle on which the apparatus operates has remained unchanged since then. There have merely been applied thereto improvements of an electronic nature seeking to provide assistance in assessing the refractive power of the lens in the measurement zone, and also relating to the user interface. That apparatus continues to give satisfaction at the present time, in particular in terms of measurement accuracy when it is used, as generally happens, by an operator who is qualified and practiced. However taking a measurement is relatively lengthy and it cannot be delegated, without risk of error, to an operator having lesser qualifications or who is a beginner. The use of that apparatus thus tends to impact on the profitability or the quality of the work carried out by an optician.
That apparatuses is characterized by the fact that it possesses a single and stationary measurement axis on which there is arranged a narrow-field system for observing the lens. It is therefore up to the operator to manipulate the lens that is to be measured so as to bring the point of interest of the lens (the measurement point) onto the optical axis of the apparatus in order to measure one or more of the refractive characteristics of the lens. The operator can then assess the refractive power of the zone of the lens (the measurement zone) that is situated in the field of the apparatus. The measurement zone is narrow, and in practice it presents a diameter of about 8 millimeters (mm). It will be understood that under such conditions, using apparatus of that kind requires manipulations to be performed that take time and that lead to inaccuracy in the event of the measurement point of the lens not being positioned with precision on the measurement axis of the apparatus. In addition, if it is desired to make a plurality of measurements on a single lens at a plurality of points thereof, as happens for example when measuring powers and axes at reference points for near vision and for far vision on a multi-focal lens, it is necessary to increase the number of manipulations, thereby further lengthening the time required for measurement.
Another narrow-field measurement technique dedicated to measuring the refractive powers of single-vision lenses in their centers, is described in the article “Testing and centering of lenses by means of a Hartmann test with four holes”, Optical Engineering, Vol. 31, No. 7, pp. 1551-1555, Jul 1992, Malacara. Apparatus implementing the technique described in that document is described in greater detail in U.S. Pat. No. 3,880,525. That apparatus comprises:                a support arranged to receive such a lens;        on a first side of the lens support, lighting means including an optical system for generating a collimated beam of light rays directed towards the ophthalmic lens installed on said support;        on a second side of the support, means for measuring the deflection imparted by the lens on said light rays, said means comprising a projection screen onto which the shadow of the lens is projected, and means for reading and digitizing the image projected onto the projection screen; and        means for detaching light rays from the light beam and suitable for detaching a localized group of at least three non-coplanar light rays that are grouped together about a measurement axis passing through the center of the lens, so that the detached rays occupy a measurement cylinder having a diameter that is substantially smaller than the diameter of the lens under measurement.        
In practice, the ray detachment means comprise a mask perforated by a few holes, typically four holes, for passing a corresponding number of light rays as detached thereby from the beam. Unlike mapping apparatuses, the detached rays are grouped together about a stationary measurement point that coincides with the center of the lens and they are contained within a radius of a few millimeters in order to obtain an accurate value for the local power at the center of the lens. It is specified that overall measurement over a larger area of the lens, or indeed over the entire lens, would be inaccurate insofar as the refractive characteristics of the lens over the remainder of its surface differs substantially from those measured in its localized central zone. In any event, the measurement axis of that apparatus is single and stationary.
Under such conditions, it can immediately be seen that that apparatus presents limits and drawbacks. Its field of application is restricted to single-vision lenses and under no circumstances does it make it possible to measure the refractive characteristics of a lens at points other than its geometrical center, without moving the lens and the associated loss of time and risk of error that such manipulation engenders, as mentioned above for frontofocometers.
Finally, wide-field mapping measurement apparatuses are known that can be thought of as apparatuses having multiple stationary measurement axes, in that they are designed to perform a plurality of measurements of the refractive powers of the lens simultaneously at a large number of measurement points that are regularly distributed over the entire area of the lens to be measured. Such an apparatus typically implements Hartmann or equivalent tests of the kind described in the article “Hartmann test”, Optical Shop Testing, Chap. 10, pp. 367-396, I. Ghozeil, edited by D. Malacara, 1992. An example of such an apparatus is described in U.S. Pat. No. 4,641,962, for example. Those mapping apparatuses have lighting means for generating a collimated beam of parallel light rays directed towards the lens, a beam separator for separating the beams into a plurality of light rays that are regularly spaced apart for passing through the lens at a multiplicity of points distributed over the entire area of the lens, and a projection screen associated with a sensor and with measurement means. The measurement means serve to measure the deflections imparted by the lens to the various light rays, and then to draw up one or more maps of the power(s) of the lens over its entire area.
Those apparatuses are useful in particular when it is desired to obtain an overall view of the refractive characteristics of the lens and how they are distributed over various zones thereof. However they do not give entire satisfaction when used for measuring refractive characteristics at a small number of specific points of the lens, mainly because of their lack of accuracy, their slowness, and their expense. Those apparatuses measure power over the entire area of the lens, which specifically implies performing measurements using rays that are distributed over the entire lens at a pitch that is not sufficiently fine or that is inaccurately centered or aligned with the position of the measurement point under consideration. The mapping feature of the measurement also implies, for electronic processing of the measurement, large memory requirements for storing the complete map of the lens, long calculation time for drawing up the map, and deflection-measuring means that are complex or inaccurate, since such measurement means ought to perform accurate measurements of all points of the lens simultaneously. The Applicant has observed that most of the points measured are not useful in verifying the prescription characteristics of an ophthalmic lens. In particular there is an identification problem of distinguishing between the various points of impact of the light rays on the projection screen, in order to associate each of them individually with the rays from which they are derived, and in spite of the large number and the closeness of ray impact points on the projection screen. Solving this identification problem requires specific identification means to be implemented which, in order to be reliable, turn out to be relatively complex, to consume large amounts of resources (processor time in particular), and to be expensive.
Furthermore, measured deflections of the lens are mapped using a Hartmann or equivalent mask or matrix which, in order to make accurate measurement possible, ought to have a pitch that is as fine as possible. Unfortunately, having beam separator elements (patterns, microlenses, etc.) that are close together leads firstly to an increased amount of computation, and secondly to worsening the identification problem, so that solving it then requires more complex identification means to be implemented.